The present invention relates to a method for continuous treatment of a saponified ethylene-vinyl acetate copolymer (hereinafter sometimes referred to briefly as EVOH). More particularly, the present invention relates to a method for continuous treatment of EVOH by which an EVOH having a good melt-moldability with a reduced variation in characteristics (fluidity, resistance to thermal discoloration, etc.) owing to variations in treating conditions, atmospheric conditions and other factors can be produced continuously and on a steady basis (with good reproducibility).
EVOH, in general, excels in transparency, gas barrier properties, aroma retention properties, solvent resistance and oil resistance and, on the strength of those characteristics, has been used broadly in various forms such as films, sheets, and containers inclusive of bottles for the packaging of food, pharmaceuticals, industrial chemicals, agrochemicals, and other products.
Usually, the molding of EVOH is made by the melt-molding technique and various shaped articles inclusive of film, sheet, bottle, cup, tube, pipe, etc. have so far been produced and made available commercially. The melt-moldability is a factor of paramount importance and for the purpose of improving the melt-moldability and heat stability of EVOH, attempts have made to treat EVOH with acetic acid, boric acid or phosphoric acid, or a metal salt thereof. To mention a few specific examples, JP Kokai S59-192564 discloses a process which comprises dispersing EVOH in water, adding boric acid thereto and stirring the mixture; JP Kokai S62-143954 discloses a process comprising stirring an EVOH slurry in an aqueous solution containing acetic acid and calcium phosphate, JP Kokai S64-66262 discloses a process comprising immersing a pelletized EVOH in an aqueous solution containing acetic acid and an acetate salt. The present applicant also proposed a method for continuous treatment of EVOH utilizing a column reactor in JP Kokai H11-152307.
However, in the batch processes described in JP Kokai S59-192564, JP Kokai S62-143954 and JP Kokai S64-66262 referred to above, the characteristics of the treated EVOH tend to depend a great deal on the degree of precision of control over preparation of the aqueous treating solution and conditions of treatment so that a batch-to-batch variation in the treated EVOH is inevitable. For example, the accelerated discoloration of EVOH due to the heat of melt-molding (decreased resistance to thermal discoloration) or the variable melt viscosity (fluidity) leading to a variation in melt-moldability (e.g. fish eyes and other defects in the appearance of shaped articles or changes in torque of the extruder) result in a failure to obtain satisfactory shaped articles. There is, thus, room for improvement. The continuous process described in JP Kokai H11-152307 is superior to the above batch processes in the stabilization of EVOH characteristics but depending on variations in various parameters of continuous treatment (water flow, treatment time, etc.) and atmosphere (change in water temperature owing to change in external atmosphere), there remains chances for variations in the characteristics of the product EVOH. Therefore, a method for attaining EVOH having more consistent characteristics has been needed.
The present inventor made intensive investigations in the above-described state of the art and found that when EVOH (A) is treated with an acid (B) and/or a salt thereof (C) by a method which comprises feeding a saponified ethylene-vinyl acetate copolymer (Axe2x80x2) in which the acid (B) and/or salt (C) has been incorporated in advance to a column reactor from its top, simultaneously feeding an aqueous solution of the acid (B) and/or salt (C) from the bottom of the reactor to thereby effect a countercurrent contact of (Axe2x80x2) with the aqueous solution (B) and/or (C), withdrawing the excess of the aqueous solution of acid (B) and/or salt (C) from the reactor top, and continuously withdrawing the treated saponified ethylene-vinyl acetate copolymer from the reactor bottom, an EVOH having a further improved melt-moldability with reduced variations in characteristics (fluidity, resistance to thermal discoloration) can be obtained steadily and continuously.
In this specification, the EVOH substance is sometimes referred to as EVOH (A) or, briefly, as (A) and EVOH containing (B) and/or (C) is sometimes referred to as EVOH (Axe2x80x2) or, briefly, as (Axe2x80x2).